I use LAV Filters for watching MXF-Files, but in MXF files the audiochannels are stored as single mono streams. My NLE detects the files correctly as stereo.
How do I configure LAV Filters to use left and right mono streams as one stereo stream?

I use LAV Filters for watching MXF-Files, but in MXF files the audiochannels are stored as single mono streams. My NLE detects the files correctly as stereo.
How do I configure LAV Filters to use left and right mono streams as one stereo stream?

As long as the MXF file format contains no hints about its contents, I do not think it's a good idea to start implicitly poking around this unless we're having the user specifically mix it in some way. If you know of MXF providing this metadata, feel free to tell about it to ffmpeg-devel, but so far it has either been in a separate XML file (if even that!), or completely vendor-specific (we had some stuff from Vendor X that did it like this, and some stuff from Vendor Y that did it like this).

I have dealt with MXF files where you had 8 audio tracks and first two of them were used for stereo, as well as MXF files where the track had 8 channels (or 6) of audio, of which only first two were actually to be actively utilized for mixing.

nev, is there something special about VP9 decoding on certain CPUs?
Decoding YT 4k 60fps VP9 video on a Haswell CPU shows very high CPU usage, while it is much lower with mpv.
Oddly, there is also such a difference between browsers. Like with mpv, CPU usage in Firefox is quite low, while it is much higher in Chrome and Edge.
This is only the case here with Haswell (4770k), with Skylake (6700k) the differences are much smaller.

How does LAV Audio manage to send a Dolby or DTS signal when the audio renderer isn't set to allow bitstreaming?

I recently set up a new pre-processor and was disappointed with how long it took to lock onto a Dolby or DTS signal (seeking would result in a silence of three to four seconds) but did not realize until today that "allow bitstreaming" was unchecked in my audio renderer's (MPC-HC Internal Audio Renderer) settings. Doing so allowed my pre-processor to lock onto the bitstream almost instantly and there is no more silence when I seek to different positions, but I am still curious why I didn't just get silence when that option was unchecked.

Decoding YT 4k 60fps VP9 video on a Haswell CPU shows very high CPU usage, while it is much lower with mpv.
Oddly, there is also such a difference between browsers. Like with mpv, CPU usage in Firefox is quite low, while it is much higher in Chrome and Edge.

Do you have a discrete graphics card in that Haswell system?
Someone told me Firefox now used hybrid VP9 decoding on their system (Sandy Bridge + Radeon 290), maybe mpv is doing the same?

How does LAV Audio manage to send a Dolby or DTS signal when the audio renderer isn't set to allow bitstreaming?

but I am still curious why I didn't just get silence when that option was unchecked.

If bitstreaming is unchecked, LAV decodes the Dolby or DTS stream to an uncompressed n-channel wav like stream. It can do this because ffmpeg supports decoding such streams to uncompressed n-channel wav-like stream.
No magic involved, just plain decoding !